Igniter element

ABSTRACT

A heat-resistant ceramic electric igniter element has a plurality of compositions in a continuous unitary body structure including a central or igniting zone of a composition having a relatively high electrical resistance flanked by end zones of a composition having a lower electrical resistance to which the leads are attached. Gradual transitions from one composition to the other are provided which compensate for any differences in the coefficient of expansion between the two compositions and minimize migration between the two compositions.

United States Patent Crandall et al. Apr. 1, 1975 [5 1 lGNlTER ELEMENT3.607.475 9/1971 Schrewclius 156/6 1 n1 g a 5] e rs William B. crandan'eamm 3.fi6.., -/l97 Ra 317/98 Linden E. Shipley, Evanston. both v of mPIIIHUI) Lrummer-Volodymyr Y. Mayewsky Attorney, Agent, or FirmCharlesG. Mersereau [73] Assigneez Honeywell Inc., Minneapolis. Minn.

[22] Filed: Jan. 10, 1974 [57] ABSTRACT I PP 2. 5 A heat-resistantceramic electric igniter element has a plurality of compositions in acontinuous unitary body U.S v l6 SU'UCIUI'E including a central Ol'igniting 20116 of 21 com- 333/33d 431/258v position having a relativelyhigh electrical resistance 5 l] Int. Cl. i 23 7/10 flanked by end Ofcomposiion having a [58] new of Search 338/275 219/552 electricalresistance to which the leads are attached. 219/553. 252/516. 264/65.Bl/258' Gradual transitions from one composition to the other i areprovided which compensate for any differences in 56] References cued thecoefficient of expansion between the two composi- UNITED STATES PATENTStions and minimize migration between the two compo= sitions. 19929597/l96l Schrcwelius 338/330 X 3.321.727 5/l967 Schrcwclius 338/330 3Claims. 2 Drawing Figures moosoo-

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TEMPERATURE (Fl ITIIIIIIIII 0. L0

llrlllllllllll 1.5 L0 05 O DISTANCE FROM CLOSEST END (INCHES)PATENIEDAPR 1 1975 FIG.I

O O o Q 0 o 0 o 8 6 4 2 may mmnk mmn=amh DISTANCE FROM CLOSEST END(INCHES) FIG.2

IGNITER ELEMENT BACKGROUND OF THE INVENTION The present inventionrelates generally to electrical resistance-type ceramic igniter elementsfor igniting gaseous fuels and the like, and, more particularly. to animproved continuous service element utilizing a plurality of ceramiccompositions in a unitary structure.

DESCRIPTION OF THE PRIOR ART In the prior art it has long been known toutilize vari ous heat-resistant oxidation-proof ceramic materials forresistance-type electric igniter elements. It is also known that anigniter may be made utilizing a plurality of ceramic Compositions havingdifferent electrical resistivities. An example ofthat concept is shownin a patent to Schrewelius US. Pat. No. 3.321 .727 issued May 23. I967.That disclosure illustrates and describes an igniter element made up ofseparate segments of differ ent materials which are subsequently joinedtogether to provide such a composite structure.

While such structures have met with partial success. they suffer fromseveral serious drawbacks. First. at the junction point between thediverse compositions there is an abrupt compositional change which doesnot allow for inherent differences in the coefficients of expansionbetween the two materials. This inherent difference in coefficients ofexpansion resistance coupled with the fact that the higher-resistancecenter portion reaches afar greater temperature than the end portionswhen a current is applied to the structure. may result in a mechanicalfailure of the igniter element at one or more of the junction pointsafter a number of heating and cooling cycles. Second. the use ofdistinctsegments in the manufacture of such an element. necessitates separatemoldings of the sections and subsequent assembly of the element whichadds to the cost and com plexity of its manufacture.

SUMMARY OF THE INVENTION According to the present invention. theproblems associated with the prior art muIti-compositionelectricalresistance type igniter elements are solved by the provisionof a unitary structure having a gradual compositional transition fromone composition to another. This overcomes the problems created bydifferences in the coefficient of expansion and molecular migrationacross the juncture between adjacent compositions and simplifies themanufacture of the element by eliminating extra molding and assemblysteps. The heatresistant ceramic electric igniter element of the presentinvention has a plurality of compositions in a continuous unitary bodystructure including a central or igniting zone of a composition having arelatively high electrical resistance flanked. in gradual transition, byend zones of a composition having a lower electrical resistance to whichthe electrical leads are attached. Thus. when an electrical current isapplied across the element the central or igniting portion will reach amuch higher temperature than the two end portions.

The central zone is normally made of a composition having a negativetemperature coefficient of electrical resistance (an electricalresistance which decreases with an increase in temperature) and the endor lead attachment zones of a material having a positive temperaturecoefficient of electrical resistance (an electrical resistance whichincreases with an increase in temperature). One successful version ofthe igniter of the invention and which is described in the preferredembodiment. below. has a central or igniting zone having a compositionincluding from about 25 percent to about 88 percent green SiC variousmeshes. from about I percent to about 8 percent ferro-silicon. fromabout I percent to about It) percent TiO- from about I percent to about20 percent ZrO from about 5 percent to about 30 percent pyrex-type glass(defined below) and from about 5 percent to about 30 percent fusedsilica. The end zones have a positive temperature coefficient ofelectrical resistance and are made of a mixture including from about 40percent to about percent green SiC of various meshes. from about Ipercent to about It) percent of fcrro-silicon. from about I percent toabout 20 percent TiO and from about 5 percent to about 30 percentpyrex-type glass.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 illustrates a heating element made in accor dance with thepresent invention and.

FIG. 2 is a schematic representation of temperature profile producedwith the igniter of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Turning now to FIG. I. we see arepresentative illustration of the igniter of the present invention. Theig niter has a unitary body It], which may be in the shape of a hairpinas illustrated. or in any other shape desired for the particularapplication ofthe device. The unitary structure 10, is divided bycomposition into a central or igniting zone I] and two end orlead-attachment zones I2 and 13 to which electrical leads I4 and I5 maybe attached as at 16 and 17. The changes in composition between zones 12and I3 and zone II have been indi cated for illustration purposes alonglines I8 and I9 which represent a gradual change from one composi tionto the other.

In the selection of the compositions utilized for both the high and lowresistance portions of the igniter of the preferred embodiment,considerations such as cost. electrical resistance properties. life orstability. at elevated operating temperatures. and forming suitabilitieshad to be considered.

After considerable experimentation involving a large number of possiblecomponents, optimum values for a high resistance mixture for theignition zone and a lower resistance mixture for the lead-attachmentzones which would produce a long-life, relatively inexpensive and easilyfabricated igniter have been developed and can be found in Table Ibelow:

TABLE LContinued In the particular combination of components selectedfor both the lead-attachment zones and the ignition zones of the igniterof the present invention. a composite is produced which may be regardedas consisting of two intercalated networks. one of which supplies theelectrical conduction mechanism and the other bonding mechanism. Thebulk of the electrical conduction mechanism is provided by the siliconcarbide and the ferro'silicon components and the basic bonding mechanismprovided by the titanium dioxide, zirconium dioxide and glasscomponents. It has also been theorized that the oxides of titanium andzirconium do contribute some electrical conductivity to the final glassysilicate bond. especially at elevated temperatures.

To be compatible with most safety circuitry. the mixtures of componentsof the ignition zone and lead attachment zones were also selected suchthat the igni tion zone has a negative coefficient of electricalresistance with increasing temperature and the leadattachment zones havea positive coefficient ofelectrical resistance.

While it is contemplated that other possible mixtures of componentswhich provide the necessary combination of properties required for theigniter ofthe present invention including some containing MoSi or othercompounds not found in Table l. the combinations illustrated for thepreferred embodiment have been found to provide an excellent ignitercombining low power consumption with a desired temperature operat ingrange in both the ignition zones and lead attachment zones.

ln addition to providing an excellent bonding material. pyrex glassutilized in the composition of both the igniter and lead-attachmentzones of the igniter of the present invention is thought to provide anadditional advantage which is ofgreat benefit in adding to the longevityof the igniter. The elevated temperatures wherein the igniters arenormally operated, it appears that the inclusion of what amounts to afilm borosilicate glass aids in inhibiting any further oxidation of thesystem which would lead to a degradation of the igniter composition andultimate failure of the igniter. in this manner. the inclusion of thebasically borosilicatcpyrex composition seems to actually extend thelife of the igniter.

In FIG. 2 there is pictured an experimentallyobtained picture profile ofa typical igniter fabricated in accordance with the present invention.The profile illustrated was obtained by electrically energizing such anigniter allowing sufficient time for the igniter to reach a state oftemperature equilibrium. The particular profile shown in FIG. 2 wasobtained by applying 60 Hz. AC (90 v. rms and 0.34 a rms) power to anigniter similar to that of FIG. 1. in normal ignition operation, theigniter is operated at a peak ignition zone temperature of about l.200 Cbut a substantially similar temperature profile obtains.

it can readily be seen by the temperature profile of FIG. 2 that thetemperature varies from approximately 1,250" F at a point close to thetip of the igniter. The reason for the highest temperature not beingprecisely located at the tip is not fully understood; however. thatvariation is probably well within the limits of experimental error. Theeffect of the gradual transition from the lead attachment zonecomposition to that of the ignition zone is readily reflected in thegeneral slope noted in the temperature profile. As explained above. thiseliminates any abrupt change in temperature between adjacent segments ofthe igniter and prevents any problems associated with such abruptchanges.

in the fabrication of the igniter of the invention. the proper mixturesfor both the ignition and leadattachment zone are premixed in the coldstate. Quantities of these mixtures are then placed in a pressing dienormally made of graphite, in the desired shape of the igniter in amanner which allows overlap of the components as illustrated in FIG. I.The die is then raised to pressing temperature of approximately l.500 Cand the igniter is pressed in the die at a pressure of approximately5.000 psi for about 15 minutes in a well known manner. Thetemperature-pressurc-time combination utilized in the hot pressing stepalso enables the igniter to achieve the desired density of greater than99 per' cent theoretically possible density and it is this high densitywhich is theoretically responsible for much of the excellent oxidationresistance achieved by the igniter of the present invention. The formedigniters are then allowed to cool in the dies and are subsequentlyremoved and the electrical leads attached by one of several techniqueswhich are well known in the art.

As explained above the preferred method of fabricating the igniter ofthe invention is a hot pressing process. Other methods of fabricationhave been attempted with less success. Thus. some experimental ignitershave been fabricated by cold pressing and subsequent sintering. ignitersmade in that manner although exhibiting the appropriate resistivity inboth the lead-attachment zones and the ignition zone. had a highporosity which severely limited the useable life of such igniters as byoxidation. Other techniques such as chemical vapor deposition. packcementation and hot isostatic pressing were also attempted as methods tofabricate the igniter of the invention. The third technique. hotisostatic pressing is also an acceptable method, but appears to be muchmore expensive than the hot pressing tech nique described with regard tothe preferred embodiment.

The embodiments ofthe invention in which an exclusive property or rightis claimed are defined as follows:

1. A continuous unitary body formed in a single segment. said bodycontaining a plurality of heat and oxidation resistent, electricallyconductive ceramic material compositions wherein said compositionscomprise: An igniting zone having a composition exhibiting a relativelyhigh electrical resistance. zones flanking said igniting zone. saidflanking zones having a composition exhibiting a lower electricalresistance than said igniting zone; wherein the compositional transitionbetween said igniting zone and said flanking zone is a gradualtransition; and

wherein said compositions of both said igniting zone and said flankingzones comprise principly of silicon carbide and pyrex-type glass. andelectrical leads attached to said flanking zones. 2. The continuousunitary body of claim I wherein said ignition zone has a negativetemperature coefficient of electrical resistance and wherein saidflanking zones have a positive temperature coefficient olelectricalresistance.

3. The continuous unitary body of claim 1 wherein said ignition zonecomprises:

from about to about 88 percent silicon carbide.

from about i to about 8 percent term-silicon.

from about 1 to about percent of oxides from a group consisting ofoxides oititianium and zirco-

1. A CONTINUOUS UNITARY BODY FORMED IN A SINGLE SEGMENT, SAID BODYCONTAINING A PLURALITY OF HEAT AND OXIDATION RESISTENT, ELECTRICALLYCONDUCTIVE CERAMIC MATERIAL COMPOSITIONS WHEREIN SAID COMPOSITIONSCOMPRISE: AN IGNITING ZONE HAVING A COMPOSITION EXHIBITING A RELATIVELYHIGH ELECTRICAL RESISTANCE; ZONES FLANKING SAID IGNITING ZONE, SAIDFLANKING ZONES HAVING A COMPOSITION EXHIBITING A LOWER ELECTRICALRESISTANCE THAN SAID IGNITING ZONE; WHEREIN THE COMPOSITIONAL TRANSITIONBETWEEN SAID IGNITING ZONE AND SAID FLANKING ZONE IS A GRADUALTRANSITION; AND WHEREIN SAID COMPOSITIONS OF BOTH SAID IGNITING ZONE ANDSAID FLANKING ZONES COMPRISE PRINCIPLY OF SILICON CABRIDE AND PYERX-TYPECLASS; AND ELECTRICAL LEADS ATTACHED TO SAID FLANKING ZONES.
 2. Thecontinuous unitary body of claim 1 wherein said ignition zone has anegative temperature coefficient of electrical resistance and whereinsaid flanking zones have a positive temperature coefficient ofelectrical resistance.
 3. The continuous unitary body of claim 1 whereinsaid ignition zone comprises: from about 25 to about 88 percent siliconcarbide, from about 1 to about 8 percent ferro-silicon, from about 1 toabout 30 percent of oxides from a group consisting of oxides oftitianium and zirconium, from about 5 percent to about 30 percent of apyrex-type glass and from about 5 percent to about 30 percent silica;and wherein said flanking zones comprise: from about 40 percent to about75 percent silicon carbide, from about 1 percent to about 10 percentferro-silicon, from about 1 percent to about 20 percent titianiumdioxide and from about 5 percent to about 30 percent of pyrex-typeglass.